A Taxonomy of Mechanisms for Multi-Access How do multiple, independent computers coordinate access to a shared medium? There are three broad approaches: they can use a modified form of a multiplexing technique they can engage in a distributed algorithm for controlled access or they can use a random access strategy Figure 14.1 illustrates the taxonomy including specific forms of each approach © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

A Taxonomy of Mechanisms for Multi-Access Fig. 14.1 A Taxonomy of Mechanisms for Multi-Access © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

Static and Dynamic Channel Allocation Channelization refers to a mapping between a given communication and a channel in the underlying system There should be a mapping between entities and a channel is referred to as 1-to-1 and static Static channel allocation works well for situations where the set of communicating entities is known in advance and does not change © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

Controlled Access Protocols Controlled access protocols provide a distributed version of statistical multiplexing Figure 14.3 (below) lists the three principal forms: These will be discussed in the following sub-sections © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

Controlled Access Protocols Reservation - it is often used with satellite transmission It employs a two-step process in which each round of packet transmissions is planned in advance Typically, reservation systems have a central controller that follows Algorithm 14.2 © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

Controlled Access Protocols Token Passing - it is most often associated with ring topologies Although older LANs used token passing ring technology popularity has decreased, and few token passing networks remain Imagine a set of computers connected in a ring and imagine that at any instant, exactly one of the computers has received a special control message called a token When no station has any packets to send the token circulates among all stations continuously For a ring topology, the order of circulation is defined if messages are sent clockwise, the next station mentioned in the algorithm refers to the next physical station in a clockwise order When token passing is applied to other topologies (bus) each station is assigned a position in a logical sequence and the token is passed according to the assigned sequence © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

Random Access Protocols © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

Random Access Protocols Researchers at Xerox PARC created a random access protocol (1973) In 1978, a standard (also called the DIX standard) was created It is widely known as Ethernet It uses cable as a shared medium instead of broadcasting radio frequency transmissions through the atmosphere Ethernet uses three (3) mechanisms to handle collisions: Carrier sense Collision detection Binary exponential backoff © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

Random Access Protocols Binary Exponential Backoff After a collision occurs a computer must wait for the cable to become idle again before transmitting a frame Randomization is used to avoid having multiple stations transmit simultaneously as soon as the cable is idle The standard specifies a maximum delay, d, and requires each station to choose a random delay less than d after a collision occurs When two stations each choose a random value the station that chooses the smallest delay will proceed to send a packet and the network will return to normal operation In the case where two or more computers happen to choose nearly the same amount of delay they will both begin to transmit at nearly the same time producing a second collision © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

Random Access Protocols By using exponential backoff an Ethernet can recover quickly after a collision because each computer agrees to wait longer times between attempts when the cable becomes busy Even in the unlikely event that two or more computers choose delays that are approximately equal exponential backoff guarantees that contention for the cable will be reduced after a few collisions  The combination of techniques described above is known by the name Carrier Sense Multi-Access with Collision Detection (CSMA/CD) Algorithm 14.4 summarizes CSMA/CD © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

Random Access Protocols CSMA/CD does not work as well in wireless LANs because a transmitter used in a wireless LAN has a limited range A receiver that is more δ than away from the transmitter will not receive a signal, and will not be able to detect a carrier Consider three computers with wireless LAN hardware positioned as Figure 14.6 (below) illustrates  © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

Random Access Protocols Wireless LANs use a modified access protocol known as CSMA with Collision Avoidance (CSMA/CA) The CSMA/CA triggers a brief transmission from the intended receiver before transmitting a packet © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

Random Access Protocols The idea is that if both the sender and receiver transmit a message all computers within range of either will know a packet transmission is beginning Figure 14.7 (below) illustrates the sequence © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.